The guarding of large areas against intrusion always presents a considerable problem despite the large number of systems developed for accomplishing this purpose. For example, in the case of international borders, the expense of installing an effective system is a major consideration. Another persistent problem stems from the need to make a sensitive detection system which does not at the same time deliver an excessive number of false alarms attributable to spurious stimulations, for instance by induced fields from power lines or storms, by natural seismic activities, by the presence of wild animals, and by ageing and drifting of electrical circuits forming part of the system.
There are a number of systems which employ an elongated array of elements which must be excited by a specially generated signal, i.e. tuned bridge circuits, piezoresistive circuits, etc. There are field radiating systems in which disturbance of the generated field by motion therewithin is detected. There are systems in which transducers detect stress-induced strain by the magnetostrictive effect, for example Kardashian U.S. Pat. No. 3,832,704. Elongated variable reluctance transducers are employed in seismic exploration at sea, for instance, as shown in Stubblefield U.S. Pat. No. 3,375,490; Witzell, U.S. Pat. No. 3,659,257; and Kostelnicek U.S. Pat. No. 3,689,875. The present invention is an intrusion detector of the variable-reluctance strain-sensing type which seeks to provide not only an improved signal generating detector configuration, but also an improved array of individual detector units.